MX250 did not have profile for Cisco 78xx family, user need to use "Generic" profile under "Device".
"Conference" function tested not working. Other function such as "Call in/out", "Hold", "Call Waiting", "Call Transfer", "MWI", "Voicemail Button" are working fine.
Cisco 78xx IP phone will download configuraiton from TFTP server and you need to upload a configuration xml file to MX250 provision server.
Besides, you need to also upload a "Dialplan" xml to MX250 or otherwise, the Cisco 78xx IP phone could not dial.
So all you need to do is to :-
1. Create a device in MX250 for Cisco 78xx IP phone to register using "Generic" profile
2. Upload the configuration xml file to MX250 provision server
3. Upload the dialplan xml file to MX250 provision server
4. Configure the TFTP address of the Cisco 78xx IP phone set to MX250 IP address (configure using the phone setting button)
5. Reboot the Cisco 78xx IP phone and you can check the register status of Cisco 78xx from MX250 device status
Attached please find the sample of the configuration xml file and dialplan xml file.